Sheathed tubular electrical heater seal



May 26, 1959 R. D. BREMER SHEATHED TUBULAR ELECTRICAL HEATER SEAL Filed June 3, 1955 i h INVENTOR.

L fiiezarey/er zz BY H AT TORN E Y Unite States Patent SHEATHED TUBULAR ELECTRICAL HEATER SEAL Robert D. Brerner, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Application June 3, 1955, Serial No. 512,924

8 Claims. (Cl. 338-261) terminal to be sealed at one end. The leakage of air through such seals into the insulation space within the sheath deteriorates the insulation and reduces its electrical insulating properties. If air were permanently excluded from the insulation space, the life of these electric heaters would be almost unlimited. However although many different methods of sealing have been tried there appears to have been no seal which could be considered permanently air tight over several decades. Most of the seals have used some such material such as glass. The glass is susceptible of cracking and separating from the sheath during the repeated heating and cooling periods to which the heater is normally subjected. There has been difficulty in providing resilient materials and difliculty in providing a holding arrangement which will maintain a permanent seal over several decades.

It is an object of this invention to provide a simple easily made tubular sheathed electric heater having its terminal end or ends permanently sealed by an air tight seal.

It is another object of this invention to provide a simple easily made tubular sheathed electrical heater having one terminal end permanently sealed by an internal resilient heat resistant plug which is so confined within the sheath that it provides a permanent resilient air tight seal.

These and other objects are obtained in the form shown in the drawings in which three terminals are held firmly in place by two spaced crushable ceramic bushings within the sheath. The space between the two bushings is filled by a resilient bushing of heat resistant synthetic rubber. The end of the sheath is also provided with a resilient heat resistant synthetic rubber plug. The remainder of the sheath, containing coiled electrical conductors spaced from each other and from the sheath is filled with a suit- .able powdered insulating material. The sheath is then compacted with a reduction about 25% to crush the ceramic bushings and tightly compact the powdered insulating material, the synthetic plugs and the bushings.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings, wherein a preferred form of the present invention is clearly shown.

In the drawings:

Figure 1 is a vertical sectional view showing the end of a tubular sheathed electric heater embodying my invention taken along the line 11 of Figure 2;

Figure 2 is an enlarged fragmentary sectional view taken substantially along the line 2-2 of Figure l of the end of the tubular electric heater prior to the compacting step showing in section the terminal portion embodying my invention;

' Figure 3 is a similar fragmentary sectional view showing the terminal portion of the heater substantially at the end of the swaging operation; and

Figure 4 is a top view of a surface heater made according to my invention.

Referring now to the drawings and more particularly to Figure 2 my sheathed tubular electrical heater includes an outer tubular sheath 20 containing three electrical terminals 22, 24 and 26 arranged in triangular relationship within the sheath 20. These terminals each connect directly to an electrical conductor. The terminal 22 connects to a coiled electrical resistance section 28. The terminal 24 connects to a coiled electrical resistance section 30. The terminal 26 connects to a coiled electrical resistance system 32. These resistance sections preferably are connected together adjacent the opposite end of the tubular sheath 20.

The terminals 22, 24 and 26 are properly held in spaced relationship during the construction of the heater by an outer resilient synthetic rubber plug 34, an outer ceramic crushable bushing 36 of lava or chalk porcelain or other ceramic electrical insulating material that may be readily crushed to a powder. Upon the inner side of the bushing 36 is an inner resilient bushing 38 of synthetic rubber. At the inner face of the bushing 38 is a crushable ceramic bushing 40 of lava or chalk porcelain or some other ceramic electrical insulating material which may be readily crushed to a powder by a force between 1500 and 2500 pounds per square inch. The plug and these bushings are fitted together within the terminal end of the sheath 20 as shown in Figure 2. For surface heaters of electric ranges and high temperature heaters, the plug 34 and the bushing 38 are preferably made of resilient silicone rubber. For lower temperature heaters the plug 34 andthe bushing 38 may be made of Buna N synthetic rubber. The plug 34 is provided with outer radial ribs 42 in the form of three arms extending radially outward from the center or its outer surface between its terminals. These ribs 42 increase the length of the surface path between the terminals. This construction makes it practical to provide as many as three terminals at one end and seal them in such a small space.

The remainder of the sheath 20 is filled with a suitable powdered insulation 44 such as powdered magnesium oxide. The opposite end of the sheath 20 may be sealed by a metal cap which may be welded to the sheath 20 to provide a permanent metal seal. If desired however this opposite end of the sheath may be provided with one or three terminals and a sealing arrangement similar to that shown in Figure 2.

The sheath 20 and its contents are then compacted or shaped in some suitable way to compact the powdered insulation 44 as well as the bushings 36, 38 and 40 and the plug 34. Preferably, as shown in Figure 3, the terminal end is swaged down in approximately a 25% reduction by a suitable swaging machine to a smaller diameter as illustrated in Figure 3. In this swaging operation, the ceramic bushings 36 and 40 hold the terminals 22, 24 and 26 in proper position substantially equally spaced from each other and also substantially equally spaced from the outer sheath 20. During the reduction of the sheath, these ceramic bushings 36 and 40 also prevent the inward movement of the synthetic plug 34 and also the bushings 36 and 40 confine and prevent axial movement of the resilient synthetic rubber bushing 38. The outer plug 34 during the reduction is forced out of the end of the sheath 20 a short distance, thus preventing it from exerting maximum sealing pressure to close the outer .entrance to the sheath 20. The ceramic bushings.36 and 40 however remain substantially in place during their reduction and their crushing and prevent any axial flow of the inner resilient synthetic rubber bushing 38. This resilient bushing 38 therefor is permanently locked within the sheath 2%} and therefore becomes highly pressed. The force upon the bushing 38 is extremely high since the inner bushing is resilient and is entirely confined with a reduction of about 25% in volume. The resiliency of this bushing 38 causes it to press tightly against the internal surfaces of the sheath 26, the adjacent faces of the bushings 36, 40 and the adjacent surfaces of the terminals 22, 24, 26. This permanently seals the terminals 22, 24 and 26 in the adjacent end of the sheath with an air tight seal capable of resisting high pressures. The pressure is so high that a permanent seal is insured even though the synthetic rubber should lose most of its resiliency because of aging.

For a surface heater, the remaining portions of the sheath may be bent into a spiral and thence pressed into a triangular shape as shown in Figure 4 to make an excellent surface heater unit. The terminals are supported in spaced relation with each other and are permanently sealed in the end of the sheath. The bushings 36 and 40 are crushed in the swaging process and flow to and remain firmly in place holding the inner bushing 38 confined within the end of the sheath at relatively high pressure. This makes an excellent resilient seal which is not subject to cracking and leakage as compared with earlier types of seals for surface heaters.

To prevent moisture on the surface from promoting surface arcing the exposed surface of the plug 34 and the adjacent portions of the terminals 22, 24 and 26 have applied thereto a water repellent. Such a water repellent may be applied by brushing, spraying or dipping. As one example, a 2% solution of linear methylpolysiloxanes in naphthalite or trichlorethylene may be brushed or sprayed thereon to provide a water repellent surface. The particular linear methylpolysiloxanes used have a viscosity of about 1000 ctks. and a formula of They are also known as methyl silicone oils or fluids. The naphthalite or trichlorethylene readily vaporizes to leave a thin water repellent film of methylpolysiloxanes upon outer surfaces of the plug to prevent surface arcing. This provides a long moisture free surface path over the ribs 42 between the terminals 22, 24 and 26 upon the plug 34 of synthetic rubber to prevent surface arcing. If the surface is expected to remain dry, the water repellent coating may be omitted.

In accordance with the provisions of Rule 78a, reference is made to the following prior filed applications: this application is a continuation-in-part of SN. 338,464 filed February 24, 1953 (now abandoned), and SN. 505,- 960 filed May 4, 1955.

While the form of embodiment of the invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted, as may come within the scope of the claims which follow.

What is claimed is as follows:

1. An electric surface heater including an outer tubular metal sheath, an electrical terminal in one end of the sheath, an electrical conductor connected to the terminal and extending therefrom in the sheath, insulating means surrounding the conductor within the sheath, a resilient outer plug of synthetic rubber having its major portion within and tightly held in the end of the sheath surrounding said terminal, a resilient inner bushing of synthetic rubber tightly held within the sheath adjacent to but spaced from said outer plug surrounding said terminal, and a crushed ceramic bushing tightly filling the space between said outer plug and said inner bushing surrounding said terminal within the sheath.

2. An electric surface heater including an outer tubular metal sheath, an electrical terminal in one end of the sheath, an electrical conductor connected to the terminal and extending therefrom in the sheath, insulating means surrounding the conductor within the sheath, a resilient outer plug of synthetic rubber tightly held in the end of the sheath surrounding said terminal, a resilient inner bushing of synthetic rubber tightly held within the sheath adjacent to but spaced from said outer plug surrounding said terminal, and a crushed ceramic bushing tightly filling the space between said outer plug and said inner bushing surrounding said terminal within the sheath, and a second crushed ceramic bushing within the sheath on the opposite side of said inner bushing.

3. An electric surface heater including an outer tubular metal sheath, a plurality of spaced electrical terminals in one end of the sheath, an electrical conductor connected to each of the terminals and extending therefrom within the sheath, insulating means surrounding and separating the conductors within the sheath, a resilient outer plug of synthetic rubber having its major portion within and tightly held within the end of the sheath surrounding and spacing said terminals, a resilient inner bushing of synthetic rubber tightly held within the sheath adjacent to but spaced from said outer plug surrounding and spacing said terminals, and a crushed ceramic bushing tightly filling the space between said outer plug and said inner bushing surrounding and spacing and firmly supporting said terminals within the sheath.

4. An electric surface heater including an outer tubular metal sheath, a plurality of spaced electrical terminals in one end of the sheath, an electrical conductor connected to each of the terminals and extending therefrom within the sheath, insulating means surrounding and separating the conductors within the sheath, a resilient outer plug of synthetic rubber tightly held within the end of the sheath surrounding and spacing said terminals, a resilient inner bushing of synthetic rubber tightly held within the sheath adjacent to but spaced from said outer plug surrounding and spacing said terminals, and a crushed ceramic bushing tightly filling the space between said outer plug and said inner bushing surrounding and spacing and firmly supporting said terminals within the sheath, and a second crushed ceramic bushing tightly filling the space on the opposite side of the inner bushing.

5. The process of making a tubular sheath heater which includes placing a conductor with a terminal connected thereto within the sheath with a resilient outer synthetic rubber plug and a crushable ceramic bushing and a resilient synthetic rubber bushing surrounding the terminal and located side by side within the sheath, surrounding the conductor within the sheath with insulating material, and deforming inwardly the sheath until the ceramic bushing is crushed and the rubber plug and bushing tightly confined within the sheath to provide an air tight seal.

6. The process of making a tubular sheath heater which includes placing a conductor with a terminal connected thereto within the sheath with a resilient outer synthetic rubber plug and a crushable ceramic bushing and a resilient inner synthetic rubber bushing and a second crushable bushing surrounding the terminal and located side by side within the sheath, surrounding the conductor within the sheath with insulating material, and deforming inwardly the sheath until the ceramic bushings are crushed and the rubber plug is tightly confined within the end of the sheath and the rubber bushing is tightly confined between the crushed ceramic bushings within the sheath to provide an air tight seal.

7. The process of making a tubular sheath heater which includes a placing conductor means in the sheath with a plurality of spaced terminals individually connected to the conductor means placed within the sheath with a resilient outer synthetic rubber plug and a crushable ceramic bushing and an inner resilient synthetic rubber bushing surrounding the terminals and located side by side within one end of the sheath, surrounding the conductor means within the sheath with insulating material, and deforming inwardly the sheath until the ceramic bushing is crushed and the rubber plug and the rubber bushing are tightly confined within one end of the sheath to provide an air tight seal.

8. The process of making a tubular sheath heater which includes placing conductor means in the sheath with a plurality of spaced terminals individually connected to the conductor means placed within the sheath with a resilient outer synthetic rubber plug and a crushable ceramic bushing and a resilient inner synthetic rubber bushing and a second crushable bushing surrounding the terminals and located side by side within one end of the sheath, surrounding the conductor means v in the sheath with insulating material, and deforming inwardly the sheath until the ceramic bushings are crushed and the rubber plug is tightly confined within the end of the sheath and the rubber bushing is tightly confined between the crushed ceramic bushings within the one end of the sheath to provide an air tight seal.

References Cited in the file of this patent UNITED STATES PATENTS 

